The present invention relates to a metal oxide semiconductor (MOS) transistor and, more particularly, to a high voltage MOS transistor.
To establish an improved high voltage MOS transistor by preventing field concentration near an edge of a gate electrode, an attempt has b een mde in which there has been additionally provided a high resistant layer adjacent a drain as a part of the drain region, the conductivity type of the high resistant layer being the same as the drain.
The conventional transistor comprises a P type substrate, an N.sup.+ type source region, an N.sup.+ type drain region, a P.sup.+ region, an N.sup.- type high resistant layer, a source electrode, a drain electrode, insulating layers, a gate electrode, field plate layers, and an additional field plate layer made of Al, polycrystalline silicon, or the like.
The P.sup.+ type region surrounds the N.sup.+ type source region for providing a gate channel for the transistor. The layer is formed by a diffusion-self-alignment process. A high voltage diffusion-self-alignment MOS transistor is described in Awane et al U.S. Pat. No. 4,058,822 issued Nov. 15, 1977, assigned to the present assignee, entitled "HIGH VOLTAGE, LOW ON-RESISTANCE DIFFUSION-SELF-ALIGNMENT METAL OXIDE SEMICONDUCTOR DEVICE AND MANUFACTURE THEREOF". The disclosure of this patent is incorporated herein by reference.
Around the N.sup.+ type drain region, the N.sup.- type high resistant layer is provided for preventing field concentration at the edge of the gate electrode. The layer constitutes a part of the drain region. Each of the source electrode and the drain electrode is composed of Al, polycrystalline silicon, or the like. Each of the field plate layers extends from each of the source electrode and the drain electrode. The gate electrode is made of Al or polycrystalline silicon, called a silicon gate.
The field plate layer which extends frrom the source electrode functions to reduce field concentration at the edges of the gate electrode. The field plate layer which extends from the drain electrode function to reduce field concentration at the boundary between the N.sup.+ type layer and the N.sup.- type layer.
If one of the field plate layers extends over a suitable limitation, a reverse field plate effect may be generated which is applied to the drain portion by the layer or to the edge of the gate electrode by the layer which extends from the source electrode. This reduces the value of a sustained voltage.
To eliminate the generation of the reverse field plate effect, the above-mentioned structure of the transistor includes a region of the N.sup.- type high resistant layer, not covered with the field plate layers, made of Al or the polycrystalline silicon. However, the amount of a sustainable voltage in the ON condition, the amount of the drain current and the value of R.sub.ON, will inevitably undesirably vary according to this structure.